Hydrangeas are a shrub that is native to Asia. The plant is best-known for a cap of lacy blooms that can sprout in shades of vivid blue or pink, depending on soil conditions. Not all hydrangeas change color, so you must first determine if you have a variety that is capable of doing so. White hydrangeas will not turn blue or pink. But if you have pink hydrangeas, you can take some steps to make them pinker.
Start with an already pink hydrangea plant. Hydrangeas turn blue in response to aluminum in the soil. It is easier to make a hydrangea blue by adding aluminum to the soil than it is to turn it pink by shielding it from aluminum present in the soil.
Add an aluminum-free granulated fertilizer with a high level of phosphorous such as a (25-10-10) fertilizer by scratching it into the soil with a cultivator around the dripline of the plant. Hydrangeas produce more vivid blossoms if they are healthier. Phosphorous will also naturally shield hydrangeas from aluminum.
Add dolomitic lime to the soil by scratching into the soil with a cultivator around the dripline. Dolomitic lime will raise the pH of the soil by making it more alkaline. This will inhibit the plant's ability to take up aluminum from the soil. Repeat this process up to four times yearly.